1. Field of the Invention
The present invention generally relates to apparatus for determining the rotational position of a body, and more particularly, it concerns an apparatus for determining when the front wheels of a motor vehicle are rotatively oriented in desired steering positions.
2. Description of the Prior Art
It is well-known that proper caster angle settings of the front wheels of a vehicle are requisites to the directional stability of the vehicle. For example, if the front wheels of a car have positive caster angles which are set too high, turning corners will require more effort and front wheel shimmy may occur at slow speeds. If the caster angles are set too low, high speed wander may result. If the caster angle of one of the front wheels is set to an excessively high positive amount relative to the caster of the other front wheel, the car will pull to the side having the least positive setting. In view of the criticality of proper caster angle settings, procedures and equipment have been developed to determine the caster angles of vehicle-mounted wheels.
The basic procedure for determining the caster angle of a front wheel of a vehicle is as follows: first, measuring the camber angle of the wheel after the wheels have been turned to a first steering position rotated in one direction from the straight-ahead position (the straight-ahead position being the position wherein the wheels are arranged such that the vehicle, when driven, will move straight ahead); second, measure the camber angle of the wheel after the wheels have been turned to a second steering position rotated in the opposite direction and angularly displaced by a preselected turn angle, such as 20.degree., from the first steering position; and third, multiply the difference between the readings by an appropriate factor to determine the caster angle of the wheel. It will be recognized that the accuracy of the caster angle as determined by this procedure is dependent upon both the accurate measurement of the two camber angles of the wheel and the accurate determination of when the wheels are in the two desired steering positions.
In the prior art, a popular apparatus for determining the turn angle of a vehicle-mounted wheel for making caster meausurements has been the turntable upon which the wheel is supported, said turntable having a protractor affixed thereto for indicating the amount of rotation in a horizontal plane of the wheel. A shortcoming of the use of a turntable is that such means fails to automatically establish a reference "zero" position when the front wheels are positioned in the straight-ahead orientation. It is a common practice to align the front wheels in the approximate straight-ahead position by sighting down the sides of the tires of the front wheels supported by the turntables and rotating the front wheels into positions such that they are equally angularly disposed with relation to the positions of the rear wheels directly therebehind. After such approximation of a straight-ahead position is accomplished, the protractor device on one or both of the turntables is adjusted to indicate zero degrees.
A major problem of the turntable is that the protractor thereon is subject to much abuse from contact with vehicle wheels and so must be ruggedly made; thus, it is often not capable of reading the turn angle with sufficient resolution, such as .+-.1.degree., necessary for making accurate caster angle measurements.
It will also be recognized that the turntable cannot be utilized when the vehicle is resting on a hydraulic jacking device and the front wheels are thus elevated from the ground.
Alternative devices for determining the steering positions are illustrated in prior U.S. Pat. No. 2,496,324 to Wilson, U.S. Pat. No. 2,641,957 to Vandermeer, and U.S. Pat. No. 2,667,805 to Carr. These patents generally disclose wheel alignment apparatus which include a pair of reflectors respectively mounted to the front wheels of a motor vehicle. Each of the reflectors has a central mirror segment which is parallel to the plane of the associated wheel, a front mirror segment extending forwardly from the central segment and forming a selected angle therewith, and a rear mirror segment extending rearwardly from the central segment and also forming said selected angle with the central segment. All of the mirror segments are planar, and the selected angle at which the front and rear segments are angularly displaced equals the preselected turn angle to the two desired steering positions from the straight-ahead position mentioned hereinbefore in connection with the caster angle determining procedure. The alignment apparatus further includes a pair of targets (or screens) situated in front of the vehicle and a pair of projectors respectively positioned at fixed locations beside the wheel-mounted reflectors. The alignment apparatus still further comprises a pair of stationary mirrors respectively associated with each projector. In operation, a light beam from a projector first strikes one of the mirror segments of the reflector mounted on the wheel, is reflected to the associated stationary mirror, and is then reflected toward the associated target or screen. Three steering positions are determined by this type of system in accordance with the reflective spacial orientation of the segmented reflector. A straight-ahead position is indicated when the beam from the projector strikes the central segment of the wheel-mounted reflector and is reflected to the target. The two steering positions at which the camber angle readings are made according to the aforedescribed caster angle determining procedure are indicated when the beam strikes the front and rear segments of the wheel-mounted reflector and is similarly reflected to the target.